Driven by stringent regulatory requirements in the United States and Europe, increasingly severe specifications have been imposed to diesel fuels, particularly with respect to sulphur content and in some areas aromatic content. In United States, sulphur content in diesel fuel was limited to 0.05 weight percent (500 ppm) starting from 1993. Environmental Protection Agency (EPA) in United States would enforce further reduction in the near future as it has targeted sulphur content in diesel fuel as low as 0.0015 weight percent (15 ppm) by 2006.
Although the effort to reduce sulphur content in diesel fuel is seen as a positive step to protect our environment, the resultant fuel's performance is less satisfactory. Hydrotreating (hydrogenation) process which is use to reduce sulphur content in diesel fuel would also cause a reduction in fuel lubricity. Usage of diesel fuel with reduced lubricity may cause injection pump failures and accelerated engine wear.
Lubricity has been included into the automotive diesel fuel standard EN590 since 1999. It estimates a fuel's ability to protect fuel injection pumps against extra wear. Fuel Injection Equipment (FIE) Manufacturers adopted high frequency reciprocating rig (HFRR) test and all diesel fuels are recommended to meet a limit of 460 micron maximum wear scar diameter. A lower wear scar diameter indicates better lubricity.
European Patent Application No. 635 558 discloses a gas oil composition, with sulphur content of not more than 0.2 percent by weight (2000 ppm) and with aromatic hydrocarbons content of lower than about 30 percent by weight, containing, as a lubricity improver agent, an amount of 0.01 weight percent (100 ppm) to 1 weight percent (10000 ppm) of lower C1-C5 alkyl esters of a mixture of saturated and unsaturated C12C22 fatty acids, derived from vegetable oleaginous seeds. The vegetable oleaginous seeds are particularly rapeseed, sunflower and soybean seeds. The mixture of C12-C22 fatty acids contains from 5 to 20 weight percent of saturated fatty acids, from 70 to 95 weight percent of total mono-unsaturated and di-unsaturated fatty acids, and from 0 to 10 weight percent of total tri-unsaturated and tetra-unsaturated fatty acids.
U.S. Pat. No. 5,993,498 discloses a polyol ester distillate fuel additive wherein said ester is characterized as having about 1% to 35% unconverted hydroxyl groups or having a hydroxyl number of greater than about 5 to 140 and wherein said ester is a reaction product of a polyhydric alcohol with at least one branched and/or linear saturated monobasic acid or a reaction product of a polybasic acid with monohydric alcohol. It is observed that polyol esters having hydroxyl number of lower than 5 do not function well as lubricity additive.
U.S. Pat. No. 6,511,520 discloses a lubricity additive for diesel and aviation fuels with low sulphur content which comprises 5 to 25 weight percent of at least one glycerol monoester, 0.1 to 20 weight percent of at least one glycerol diester and 35 to 75 weight percent of at least one compound of formula R2—C(O)—X, R2 being an alkyl chain containing 8 to 24 carbon atoms, or a monocyclic or polycyclic group comprising 8 to 60 carbon atoms, and X being selected among (i) the groups of OR0, R0 being a hydrocarbon radical comprising 1 to 8 carbon atoms, optionally substituted by one or several esters; and (ii) the groups derived from primary or secondary amines and alkanolamines with aliphatic hydrocarbon chain, comprising 1 to 18 atoms.
One of the major disadvantages of the esters as disclosed above is their low lubricating power at a concentration of less than 0.5 weight percent in fuels with ultra low sulphur content, such as Class 1 Swedish diesel fuel.